Wind turbines, which harness the wind to produce power, represent an ever-increasing source of electrical energy. Basically, the wind impinges against vanes on a rotor, causing the rotor to revolve, and the rotor powers an electrical generator. But the vanes need considerable length and surface area to capture the wind, and as a consequence they rotate at a relatively low angular velocity. The generator must operate at a high angular velocity if it is to remain relatively compact.
To satisfy these ends, the typical wind turbine includes a transmission between the rotor and the generator to step-up the speed of rotation. The gearing of the transmission includes a planetary set which the rotor drives, an intermediate shaft which the planetary set drives, and an output shaft which the intermediate shaft drives. The output shaft is connected to the generator. Wind turbines must operate at low sound levels, and to reduce the noise produced by the gearing on the higher speed intermediate and output shafts, the gears on those shafts are cut with helical teeth. Helical gears induce thrust loads which the bearings for the shafts must accommodate. Indeed, the bearings for each shaft must accommodate thrust loading in both axial directions as well as radial loading. In this regard, most of the time the rotor drives the generator and this produces thrust loads in each shaft in only one direction. However, during some service procedures, the generator is converted into a motor which produces thrust loads in each shaft in the opposite direction, although of a lesser magnitude.
Bearings at two positions usually support the intermediate shaft and also the output shaft. These bearings are organized in a locating-nonlocating configuration or a cross locating configuration.
In the locating-nonlocating configuration bearings at one position confine its shaft axially and further carries radial loads. At the other position a bearing merely takes radial loads. The nonlocating bearing, since it does not take thrust loads, accommodates thermal expansion and contraction between its shaft and the housing without inducing axial loads in any of the bearings. Typically the locating position includes (FIG. 1) a deep groove ball bearing used in conjunction with a single-row cylindrical roller bearing (NU style—radial loading only), although two-row spherical roller bearings and a two-row cylindrical roller bearings (NJ style—radial and axial loading) have also been used at this position. Deep groove ball bearings can experience unusual wear patterns and can fail prematurely along their raceways. Hence, transmissions containing these bearings are constructed such that the bearing and their shafts can be easily removed. Two-row bearings, moreover, consume considerable axial space where space is at a premium.
In the cross locating configuration, the bearings at both positions accommodate the axial loads, with one taking the gear thrust in the primary direction, that is when the rotor drives the generator, and the other taking gear thrust in the secondary direction, which the generator serves as a motor to drive the rotor. Typical of these bearings are single row tapered roller bearings mounted in opposition. Sometimes two cylindrical rollers bearings, each ribbed on both of its races to take thrust loads (NJ style), are used. Either of these cross-locating designs must be carefully adjusted at assembly to insure that the proper radial and axial clearances are present in the bearings and bearing system. These clearances are critical in terms of controlling the load zones in the bearings, the heat generation of the system, and the final position of the gear contacts under the operating loads and temperatures. Although adjustment of these clearances is a common and well-acknowledged practice in the transmission industry, the concern always remains in this industry over manufacturing errors or operator error during adjustment, altering the expected settings and subsequently affecting the performance of the transmission (high temperatures, poor gear contact, noise vibration, etc).